1) Field of the Invention
The invention relates to a process for the preparation of optically active carboxylic acids of the formula I* or II* ##STR2## in which X is an oxygen or sulfur atom and n is 1 or 2.
2) Description of the Related Art
For the preparation of optically active carboxylic acids by resolution of racemates, up until now the racemic carboxylic acid to be separated was customarily reacted with an optically active amine and the diastereomeric salts formed were separated by fractional crystallization. This method was also used in the preparation of some of the abovementioned optically active heterocyclic carboxylic acids.
Thus, Belanger et al (Can. J. Chem. 61 (1983) 1383) describe a process for the preparation of (R)- or (S)-tetrahydrofuran-2-carboxylic acid (formula II*: n=1, X.dbd.O with the aid of brucine or cinchonidine. However, these reagents cannot be employed because of their toxicity and their expense for obtaining relatively large amounts of the substance. Cervinka et al. (Collect. Czech. Chem. Commun. 51 (1986) 404) describe the resolution of the racemate of tetrahydrofuran-2-carboxylic acid with quinine. Using this method, however, only the (S)-enantiomer is obtained, whose yield after the required repeated recrystallization is only about 10%. In JP-A 01/216983, the resolution of the racemate of tetrahydrofuran-2-carboxylic acid using optically active 4-bromo- or 4-chloro-1-phenylethylamine is described. However, the origin of these optically active amines is not known.
The resolution of the racemate of tetrahydrothiophene-2-carboxylic acid (formula II: n=1, X.dbd.S) using brucine or cinchonidine (Claeson et al, Ark. Kemi 26 (1967) 247; Stork and Kreft III, J. Am. Chem. Soc. 99 (1977) 3851) and also using N-methyl-(S)-1-phenylethylamine (Cervinka et al., Collect. Czech. Chem. Commun. 51 (1986) 404) in each case only yielded one of the two enantiomers in moderate yields The same is true for the resolution of the racemate of (2H)-tetrahydropyran-2-carboxylic acid (formula II: n=2, X.dbd.O with quinine ((S)-enantiomer in 4 % yield, Cervinka et al., Collect. Czech. Chem. Commun. 51 (1986) 404), the resolution of the racemate of tetrahydrofuran-3-carboxylic acid (formula I: n=1, X.dbd.O with quinine (Katsura, Chem. Abstr. 56 (1962) 9950; Kaneko et al., Chem & Ind. 1960, 1187; Hill et al. J. Org. Chem. 27, (1962) 921) and the resolution of the racemate of tetrahydrothiophene-3-carboxylic acid (formula I: n=1, X.dbd.S) using N-methyl-(S)-1-phenylethylamine (Cervinka et al., Collect. Czech. Chem. Commun. 51 (1986) 404).
The above disadvantages stand in the way of the preparation of relatively large amounts of substance and restrict the possibility of using optically active heterocyclic carboxylic acids as synthetic building blocks, for example in liquid crystals (cf. EP-A 355561). Moreover, until now (2H)-tetrahydrothiopyran-2-carboxylic acid (formula II: n=2, X.dbd.S), (2H)-tetrahydropyran-3-carboxylic acid (formula I: n=2, X.dbd.O and (2H)-tetrahydrothiopyran-3-carboxylic acid (formula I, n=2, X.dbd.S) have not yet been prepared in optically active form. There is, however, a need for these last-mentioned hitherto unknown carboxylic acids, as these can also be used as building blocks for liquid crystals because of their structural relationship with the abovementioned known carboxylic acids.
The object of the invention was therefore to make available a simple process for obtaining the optically active compounds with the formulae I* and II*, with which the optical antipodes can be obtained in good yields in a simple manner.